Abstract

A new occurrence of conical and branched pseudocolumnar stromatolites in Archean dolostones in the Pilbara region, Australia, contributes significant new morphologic information on such structures. These remains are interpreted as probably representing, in part, microbially mediated accretionary growth surfaces in an Archean hypersaline depositional basin. The structures comprise laterally linked pseudocolumns of centimeter width and decimeter height, with first-order conical laminae of as much as 15 cm of synoptic relief and apical angles of 30°–80°. The conical laminae are modified by a second-order, centimeter-scale, low-amplitude primary corrugate lamination, with crests and troughs occasionally stacked to form satellitic, obliquely directed pseudocolumns; bedding surfaces exhibit a preferred direction of elongation of the cones, an orientation that is orthogonal (and unrelated) to the trend of younger folding; the microstructure is secondary. The stromatolites are better preserved than those previously known from chert in the Warrawoona succession. The remains exhibit certain distinct morphologic attributes corresponding to those in younger stromatolites, such as displayed by Thyssagetes and Jacutophyton, whose biogenicity is generally accepted (although difficult to demonstrate conclusively); the conical Warrawoona forms may represent the oldest known precursor of these taxa.